I live in Portugal at the moment and wildfires are an increasing concern. Which has had me thinking about how one would go about hardening a WikiHouse design against wildfires. Here’re a few thoughts after reading various online resources, with one assumption being that you’re designing a detached building.
Site layout and defensible space
- Situate the building as far as you can from adjacent woodland and place incombustible material between—such as patios, pools, driveways and low fire-retardant plants—while paying particular attention to areas downhill of the building.
- Provide sufficient access for fire tender vehicles, preferably with a turning area.
Protect against embers
- Embers can float into cavities and openings and ignite a fire.
- Use ember-resistant vent grilles (they’re protected with a micro filter metal mesh) and/or consider fire dampers which can be closed manually or by a fusible link that melts at a certain temperature.
- Use cladding and roofing finishes that don’t leave gaps. Use sheet materials without gaps or snugly interlocking tiles. Rainscreen venting should be protected with fine metal insect mesh that doubles up as protection against embers.
- Avoid gaps between bottom of external wall and ground level, to ensure embers do not make their way under the building.
Fire resistant envelope
- Consider incombustible cladding materials, such as brick, adobe, stone, clay tile, slate, metal and incombustible sheet materials (such as cement board, calcium silicate, gypsum fibre boards) when cladding walls, roofs and soffits. These choices will need to be balanced against local availability of materials and the carbon footprint of transporting heavy materials.
- Steeper roofs will encourage embers to roll off and away from the building. Flat roofs will retain them.
- A well designed and well maintained green roof can provide protection against wildfire. See more here
- Consider adding mineral wool sheathing insulation. Sheathing insulation—insulation fitted to the external face of the WikiHouse frame before any cladding—will be a consideration if you’re aiming for Passivhaus performance, but it could also be used to protect the main structure against heat and fire.
- Glass will typically fail quickly in extreme heat. Double or triple glazing with tempered glass will fair best. But the best form of protection is to install external metal roller shutters to all windows, which can be closed manually or by a fusible link that melts at a certain temperature.
- Install fire-rated external doors and/or consider metal roller shutters as above.
- Consider melt-resistant or incombustible infill insulation. If a fire has made its way beyond the breather membrane then it’s probably just a matter of time before fire makes its way inside, whether you’ve used incombustible insulation or not. But by using an incombustible—or at least a melt-resistant infill insulation—you may guard against damage to the main structure in the event of extreme heat.
- Consider an intumescent breather membrane, while bearing in mind that these are designed to increase the time for building occupants to escape, rather than protect the building from permanent damage. If the intumescent membrane is activated as the result of a wildfire it’s probably just a matter of time before fire makes its way inside. But it could potentially act as the final line of protection that protects the main structure from critical damage, leaving you with a building that require repairs to the external finishes only, rather than the structure as well.
- Consider installing a sprinkler system with heads on the roof, over decks, patios and balconies. Preferably powered by off-grid source of power.